This invention relates to a SOI substrate having a buried oxide layer disposed near the surface of a silicon substrate and a single crystal silicon layer [hereinafter referred to as xe2x80x9cSOI (silicon-on-insulator) layerxe2x80x9d] superposed thereon. More particularly, it relates to a SOI substrate obtained by the SIMOX (Separation by IMplanted OXygen) technology and a method for the production thereof.
As the SOI substrate having a single crystal silicon layer formed on such an insulation material as silicon oxide, SIMOX wafers and bonded wafers have been mainly known. The SIMOX wafer is a SOI substrate which is obtained by implanting oxygen ions into the interior of a single crystal silicon substrate and subsequently performing an annealing treatment on the substrate thereby inducing a chemical reaction between the oxygen ions and the silicon atoms in the substrate and consequently giving rise to a buried oxide (BOX) layer in the substrate. The bonded wafer is a SOI substrate which is obtained by joining two single crystal silicon wafers across an interposed oxide layer and then transforming either of the two wafers into a thin film.
The MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) which is formed in the SOI layer of such a SOI substrate is capable of repressing the short channel effect which becomes more severe as design rule of device fabrication process becomes smaller, and allowing a saving in the power consumption involved in the operation thereof in addition to excelling in radiation hardened characteristics and avoidance of latchup and exhibiting high reliability. Further, it is capable of exalting the signal transmission speed and realizing high speed operation of the device because the operating region of the device is electrically isolated from the substrate itself. By this reason, the SOI substrate acquires a prospect of becoming a high-performance semiconductor substrate for the application of MOS-LSI in the next generation.
Among other species of the SOI substrate, the SIMOX wafer possesses the characteristic feature of particularly excelling in the uniformity of thickness of the SOI layer. In the SIMOX wafer, the SOI layer can be formed in a thickness of not more than 0.3 xcexcm and also the SOI layer having a thickness in the neighborhood of or even not more than 0.1 xcexcm can be controlled satisfactorily in thickness. Particularly, the SOI layer measuring not more than 0.1 xcexcm in thickness is often utilized in the formation a MOS-LSI adapted for the fully depleted type operation. Since, in this case, the thickness of the SOI layer itself has proportionality with the threshold voltage of the MOSFET operation, the uniformity in thickness of the SOI layer constitutes itself an important factor for the manufacture of a high-performance device in a high yield. From this point of view, the SIMOX wafer which excels in the uniformity of thickness of the SOI layer acquires a bright prospect of becoming a substrate for use in the MOSFET of the next generation.
In the manufacture of a SIMOX substrate, the implantation of oxygen ions is generally effected by the use of a single accelerating energy, typically an energy approximating closely to 200 keV. It is well known that, in this case, only when the amount of oxygen ions to be implanted is either in a region of not less than 1.5xc3x971018 pieces/cm2 or in a limited region in the range of 2.5-4.5xc3x971017 pieces/cm2, the SIMOX structure obtained after a heat treatment at an elevated temperature is enabled to acquire a buried oxide layer of satisfactory continuous and uniform quality (as disclosed in S. Nakashima and K. Izumi, Journal of Materials Research, Vol. 8, 523 (1993), for example). As respects these species of SIMOX substrate that are manufactured by implanting oxygen ions in such amounts as mentioned above, it is customary to designate the species of SIMOX manufactured by implanting oxygen ions in an amount in the former range as high-dose SIMOX substrates and the species of SIMOX manufactured by implanting oxygen ions in an amount in the latter range as low-dose SIMOX substrates.
The high-dose SIMOX substrates and the low-dose SIMOX substrates respectively possess specific characteristics of their own and find utility in particular applications which are appropriate for these specific characteristics. Of these species, the low-dose SIMOX substrates acquire a prospect of decreasing the threading dislocation density in the surface silicon layer and realizing low production cost as well because the amount of oxygen ions to be implanted therein is comparatively small. The low-dose SIMOX substrates meanwhile have the problem of leak defects generation in the buried oxide layer and insulation resistance deficiency with a high probability because of the small thickness of the buried oxide layer.
As a measure to contribute to the improvement of the quality of the buried oxide layer of the low-dose SIMOX substrate, the internal thermal oxidation process (hereinafter referred to as an xe2x80x9cITOX processxe2x80x9d) has been proposed (U.S. Pat. No. 5,658,809 and U.S. Pat. No. 5,918,136 or S. Nakashima et al., Journal of Electrochemical Society, Vol. 143, page 244 (1996)). According to the ITOX process, the treatment of oxidation at an elevated temperature induces growth of a thermal oxide film on the surface of a substrate and growth of an oxide film in a certain amount on the upper interface of a buried oxide film as well and consequently renders addition to the thickness of a buried oxide film possible. It is reported that this process produces the consequence of feasibilizing both the repression of leak defects and the improvement in insulation resistance in the buried oxide layer.
The surface silicon layer in the low-dose SIMOX substrate still contains threading dislocations at a density in the approximate range of 102-104 pieces/cm2 or in a higher range, though repressed as compared with the high-dose SIMOX substrate. It has been pointed out that when the ITOX treatment (internal oxidation treatment) is carried out at a generally adopted elevated temperature of about 1350xc2x0 C. in the process for the production of such a low-dose SIMOX substrate, depressions measuring approximately 2 xcexcm in diameter and 10 nm in depth and centering around the sites of threading dislocations are generated on the surface of the SOI layer (W. P. Maszara et al., Proceedings 1997 IEEE International SOI Conference, page 18). Typically, the fully depleted type device using a SOI of a thickness of not more than 100 nm has the threshold voltage of its operation vary with the thickness of the SOI layer. Since the depressions mentioned above affect the local variation of the SOI thickness, the fully depleted type device which is manufactured on such a substrate has the possibility of imposing a restriction on the performance of operation thereof.
In the ITOX technique mentioned above, since the effect of the internal oxidation which is relied on to produce an increment in the thickness of the buried oxide film induces growth of a surface oxide film in a thickness of not less than 10 times the increment in the buried oxide film, the surface silicon layer inevitably has the thickness thereof decrease. An effort to secure an increment in the buried oxide film owing to the effect of internal oxidation for the purpose of improving the quality of the buried oxide layer has no alternative but to decrease the thickness of the surface silicon layer, with the result that the produced silicon layer will impose a restriction on the thickness thereof. Otherwise, an effort to secure a prescribed surface silicon layer in the eventually produced SIMOX structure entails the necessity for limiting the amount of an oxide on the surface of the substrate, with the result that the degree with which the buried oxide layer is improved in quality will have its own limit.
The low-dose SIMOX substrate manufactured by the use of the ITOX process, though improved in the quality of the buried oxide layer by the ITOX effect as described above, suffers generation of depressions approximately measuring 2 xcexcm in diameter and 10 nm in depth at the sites of threading dislocations which persist in the surface silicon layer. When a fully depleted type device using a SOI layer of a small thickness typically not more than 100 nm is formed on such a substrate, therefore, the device entails the problem of possibly imposing a restriction on the improvement of the performance of its operation because the threshold voltage of operation thereof is affected by the local variation of the thickness of the SOI layer.
This invention, by repressing in the conditions for the thermal treatment performed in the method for producing a SIMOX substrate the oxygen concentration in the ambient air to the extent of preventing the ITOX effect from manifesting, makes it possible to moderate the aforementioned restriction on the conventional ITOX, lessen the leak defects in the buried oxide layer, and allow provision of a SIMOX substrate of higher quality.
Further, this invention, by elaborately defining the conditions for the thermal treatment involved in the method for producing a SIMOX substrate, makes it possible to overcome the problems encountered by the conventional ITOX and allow provision of a SIMOX substrate of higher quality and a method for the production thereof.
The objects mentioned above are accomplished by the following items (1)-(20).
(1) A method for the production of a SIMOX substrate having a buried oxide layer and a surface single crystal silicon layer formed therein by implanting oxygen ions into a silicon single crystal substrate and subsequently performing a heat treatment at an elevated temperature on the substrate, which method is characterized by performing the former stage of the heat treatment at a temperature of not lower than 1150xc2x0 C. and lower than the melting point of single crystal silicon in an atmosphere obtained by adding oxygen under a partial pressure of not more than 1% to an inert gas and subsequently performing at least part of the latter stage of the heat treatment by increasing the partial pressure of oxygen within a range in which no internal oxidation is suffered to occur in the buried oxide layer.
(2) A method set forth in (1) above, wherein the temperature of the latter stage of the heat treatment is not lower than 1150xc2x0 C. and falls short of the melting point of single crystal silicon.
(3) A method set forth in (1) above, wherein the temperatures respectively of the former stage and the latter stage of the heat treatment are each not lower than 1300xc2x0 C. and fall short of the melting point of the single crystal silicon.
(4) A method set forth in (1) above, wherein at least part of the latter stage of the heat treatment is performed in an atmosphere formed by adding oxygen under a partial pressure in the range of 1-10% to an inert gas.
(5) A method set forth in (1) above, wherein the temperatures respectively of the former stage and the latter stage of the heat treatment are each not lower than 1350xc2x0 C. and falls short of the melting point of the single crystal silicon, and at least part of the latter stage of the heat treatment is performed in an atmosphere formed by adding oxygen under a partial pressure in the range of 1-10% to an inert gas.
(6) A method for the production of a SIMOX substrate having a buried oxide layer and a surface single crystal silicon layer formed therein by implanting oxygen ions into a silicon single crystal substrate and subsequently performing a heat treatment at an elevated temperature on the substrate, which method is characterized by performing the former stage of the heat treatment at a temperature of not lower than 1150xc2x0 C. and lower than the melting point of single crystal silicon in an atmosphere obtained by adding oxygen under a partial pressure of not more than 1% to an inert gas and subsequently performing at least part of the latter stage of the heat treatment at a temperature of not higher than 1300xc2x0 C. so as to induce an internal oxidation in the buried oxide layer.
(7) A method set forth in (6) above, wherein the temperature of the former stage of the heat treatment is not lower than 1300xc2x0 C. and falls short of the melting point of the single crystal silicon.
(8) A method set forth in (6) above, wherein the temperature of the treatment for internal oxidation is in the range of 1150xc2x0-1280xc2x0 C.
(9) A method set forth in (6) above, wherein the temperature of the treatment for internal oxidation is in the range of 1150xc2x0-1250xc2x0 C.
(10) A method set forth in (6) above, wherein the partial pressure of oxygen in the treatment for internal oxidation is not lower than 50%.
(11) A method set forth in (6) above, wherein the temperature of the former stage of the heat treatment is not lower than 1300xc2x0 C., the temperature of the subsequent treatment for internal oxidation is in the range of 1150xc2x0-1280xc2x0 C., and the partial pressure of oxygen is not lower than 50%.
(12) A method for the production of a SIMOX substrate having a buried oxide layer and a surface single crystal silicon layer formed therein by implanting oxygen ions into a silicon single crystal substrate and subsequently performing a heat treatment at an elevated temperature on the substrate, which method is characterized by performing the former stage of the heat treatment at a temperature of not lower than 1150xc2x0 C. and lower than the melting point of single crystal silicon in an atmosphere obtained by adding oxygen under a partial pressure of not more than 1% to an inert gas, subsequently performing a heat treatment by increasing the partial pressure of oxygen in a range in which the buried oxide layer is not allowed to induce internal oxidation, and thereafter using, during the generation of internal oxidation in the buried oxide layer induced by increasing the partial pressure of oxygen and performing a heat treatment, a temperature of not higher than 1300xc2x0 C.
(13) A method set forth in (12) above, wherein the temperatures respectively of the former stage of the heat treatment and the subsequent heat treatment are each not lower than 1300xc2x0 C. and fall short of the melting point of the single crystal silicon.
(14) A method set forth in (12) above, wherein the temperature of the treatment for internal oxidation is in the range of 1150xc2x0-1280xc2x0 C.
(15) A method set forth in (12) above, wherein the temperature of the treatment for internal oxidation is in the range of 1150xc2x0-1250xc2x0 C.
(16) A method set forth in (12) above, wherein the partial pressure of oxygen in the treatment for internal oxidation is not lower than 50%.
(17) A method set forth in (12) above, wherein the temperatures respectively of the former stage of the heat treatment and the subsequent heat treatment are each not lower than 1300xc2x0 C. and lower than the melting point of the single crystal silicon, the temperature of the subsequent treatment for internal oxidation is in the range of 1150xc2x0-1280xc2x0 C., and the partial pressure of oxygen is not lower than 50%.
(18) A method set forth in (12) above, wherein the temperatures respectively of the former stage of the heat treatment and the subsequent heat treatment are each not lower than 1300xc2x0 C. and lower than the melting point of the single crystal silicon, the temperature of the subsequent treatment for internal oxidation is in the range of 1150xc2x0-1250xc2x0 C., and the partial pressure of oxygen is not lower than 50%.
(19) A SIMOX substrate produced by a method set forth in any of (1)-(18) above.
(20) A SIMOX substrate comprising a surface single crystal silicon layer having a thickness in the range of 10-120 nm and a buried oxide layer having a thickness in the range of 80-200 nm and characterized by the undulation of the surface of the substrate having an amplitude of not more than 8 nm.
In the present invention, in the heat treatment carried out subsequent to the heat treatment of the first step, the treatment is carried out under the condition increasing the partial pressure in the range that ITOX effect does not generate than that of the first step. Although the ITOX effect does not generate under such condition, in other words although thickness of the buried oxide layer does not increase, the leak defects of the buried oxide layer decrease along with the treatment time. On the other hand, in this process, comparing with the conventional ITOX process wherein not less than 30% of the oxygen partial pressure, lower oxygen partial pressure atmosphere is used, so growth of surface oxide film during the heat treatment is surpressed compared to the conventional ITOX treatment process. Therefore, it becomes possible to maintain that longer heat treatment time until reaching the desired thickness of silicon layer, and as a result, it becomes possible to decrease further leak defects of the buried oxide layer.